The present invention relates to capacitive membrane ultrasonic transducers (CMUTs). In particular, electrode structures are provided for CMUTs.
A typical 1D or 2D ultrasound transducer includes hundreds or even thousands of separate transducer elements. For a CMUT, a plurality (e.g., tens, hundreds, or thousands) of cells may be used together to form a single element.
A cell of a CMUT typically has a void (vacuum gap) covered by a membrane. An electrode is positioned on or within the membrane and another at the base of the void. The electrodes are exposed to the void or may be separated from the void by an electric insulator. To generate acoustic energy, a DC bias and an electrically varying signal are applied across the electrodes, causing the membrane to flex. To generate electrical energy, the acoustically induced flexing of the membrane generates a differential electrical signal between the two electrodes.
Very large electric fields are sustained across the vacuum gaps for the CMUT to generate transmit pressures on par with a piezoelectric transducer. For example, 1.2-1.3 billion volts per meter are required to achieve 1.5 Mega Pascal output pressures at 10 MHz. When exposed to these high electric fields, insulators can lose their insulating properties and begin to leak current. Electrons that pass into the insulator and become trapped or that are emitted from an insulator surface and strike the opposing insulator can alter the electric field in the gap. The altered electric field may degrade CMUT acoustic performance over time.
Historically, attempts to prevent charging degradation over time have focused on CMUTS where, one or both of the insulators are partially or completely removed or not provided. The conductive electrodes are exposed to the cavity. As long as the applied electric field is below the threshold of field emission for the exposed conductive surfaces, these CMUT structures experience less charging degradation over time than insulated CMUTs. However, electrical asymmetry may limit use in bipolar applications. There may be increased shorting between top and bottom electrodes, reducing device yield and raising patient safety concerns.